The present invention relates to an optical recording medium and, in particular, to an optical recording medium using a photochromic material as a record film, which ensures reliable retention of information recorded therein even if the recorded information is repeatedly reproduced and also relates to a process for reproducing information recorded in the optical recording medium.
An optical recording medium in general includes a record film, on which information is recorded by irradiating a recording light.
A photochromic material, which is used for a filter for adjusting luminous energy, a display, an actinometer, a recording medium for photo-printing process, a paint or the like, becomes also recently important in the field of an optical recording medium using laser beams as a recording light, because it has been found that the photochromic material has a possibility of being used as a reversible memory medium with a high density of information.
Recording and erasing of information in an optical recording medium, in which the photochromic material, for example, an azo compound, is used as a record film, is carried out, for example, in the following manner. Namely, a recording light having 420 nm of wavelength is irradiated to a record film, and the irradiated part in the record film is then colored to form a colored record spot corresponding to a record pit. Thereafter, if an erasing light having 550 nm of wavelength is irradiated, the colored record spot is discolored so that the record pit disappears.
However, reproducing of information recorded in the optical recording medium provides the following problem. If the reproducing is carried out by using the light having 420 nm of wavelength, the part of the record pit remains without disappearing but the other un-colored part is colored. As a result, the whole of the record film is colored, thereby the record pit being covered, that is,the recorded information disappears. On the other hand, if the reproducing is carried out by using the light having 550 nm of wavelength, the colored record spot is discolored, so that the recorded information disappears.
In such manner, the greatest problem of the optical recording medium in which the photochromic material is used as a record film is that the recorded information disappears in reproducing by using the above reproducing light so that it is impossible to repeatedly reproducing the recorded information.
To solve such problem, a process for reproducing the recorded information has been recently proposed, in which a mixed light comprising the light having 420 nm of wavelength (hereinafter referred to as a 420 nm light) and the light having 550 nm of wavelength (hereinafter referred to as a 550 nm light) is used for reproducing the recorded information. The principle is as follows. When the mixed light is irradiated to the record film, on which information is recorded, to reproduce the recorded information, in the colored record spot, that is, the record part, the 550 nm light is absorbed to discolor the colored record spot, thereby heat is produced. The heat promotes the coloring reaction by the 420 nm light to retain the colored record spot. On the other hand, in the part other than the colored record spot, that is, the un-record part, the 550 nm light is not absorbed, so that heat is not produced, whereby even if the part absorbs the 420 nm light and the coloring reaction occurs, thus the reaction being not promoted. As a result, if the part is slightly colored, the slightly coloring is discolored by the 550 nm light to retain the un-record part.
Even thought the mixed light is used in reproducing the information, coloring remains to a slight degree. Therefore, when the reproducing is repeated, the 550 nm light becomes absorbed by the slight coloring to produce the increased heat, so that the un-record part is colored to such a degree not so as to distinguish the record part with the un-record part.